[FEATURE] Add categories-based menu

Now that system categories are activated by default for pages,
provide a simple way of display categorized pages with a new
typee of menu for the menu/sitemap content element.

The rendering uses a new type of special HMENU called "categories",
which means that it is also possible to create categories-based
menus in pure TypoScript.

Resolves: #51161
Documentation: #51162
Releases: 6.2
Change-Id: I137158f39607ad65dff13a604210a647cb8dda6f
Reviewed-on: https://review.typo3.org/23184
Reviewed-by: Mattias Nilsson
Tested-by: Mattias Nilsson
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind